Argon and nitrogen adsorption studies of changes in connectivity of ordered cage-like large mesopores during the hydrothermal treatment

FDU-1 silicas with large cage-like mesopores were synthesized, using tetraethyl orthosilicate as a silica source and a poly(ethylene oxide)–poly(butylene oxide)–poly(ethylene oxide) triblock copolymer (EO39BO47EO39) as a structure-directing agent. The evolution of the FDU-1 pore structure during a hydrothermal treatment at 373, 393, and 413 K was studied, using nitrogen and argon adsorption at 77 K. As the time of the hydrothermal treatment was extended, a moderate increase in the pore cage diameter and a significant enlargement of the pore entrances took place, as inferred from the analysis of adsorption and desorption branches of isotherms, respectively. The materials retain a monodisperse distribution of pore cage diameters. On the other hand, argon and nitrogen desorption data provided evidences that there are two major populations of pore entrances according to their diameter and that they both undergo a concomitant enlargement as the hydrothermal treatment is prolonged. Argon desorption was superior in providing information about pore connectivity in FDU-1 samples, except for those obtained after particularly severe hydrothermal treatments (long time or high temperature). The latter materials had a fraction of pores, which were so large that the capillary condensation of argon at 77 K in them was not observed, and consequently, the information regarding the connectivity and volume of this fraction of pores cannot be extracted from the argon data. In the case of relatively large mesopores (above ∼15–20 nm), nitrogen that exhibits capillary condensation at 77 K in much wider range of pore sizes (up to 100–200 nm) is better as far as the pore connectivity assessment from desorption data is concerned.